Enantioselective catalysis using chiral metal complexes provides one of the most general and flexible methods for achieving asymmetric organic reactions. Metallic elements possess a variety of catalytic activities, and permutations of organic ligands or other auxiliary groups directing the steric course of the reaction are practically unlimited. Efficient ligands must be endowed with, for example, suitable functionality, appropriate chirality, a structure capable of differentiating space either electronically or sterically and skeletal rigidity or flexibility.
Among the asymmetric organic reactions catalyzed by chiral transition metal complexes, asymmetric hydrogenation has been one of the best studied, due in large part to the fact that it is the basis for the first commercialized catalytic asymmetric process. See, for example, ApSimon, et al., Tetrahedron, 1986, 42, 5157.
Some of the more interesting of the asymmetric hydrogenation catalysts are those derived from BINAP [2,2'-bis(diphenylphosphino)-1,1'-binaphthyl]. See, for example, U.S. Pat. Nos.: 4,691,037; 4,739,084; 4,739,085; 4,764,629; 4,994,607; and 4,766,227. Unlike the more classical models of chiral (asymmetric) molecules, chirality in the case of the BINAP-type compounds arises from the restricted rotation about the single bond joining the naphthalene rings. Isomers arising from this type of asymmetry are termed atropisomers.
BINAP-based Ru(II) and Rh(I) complexes induce high enantioselectivity in catalytic reactions. See Noyori and Takaya, Acc. Chem. Res., 1990, 23, 345.
The BINAP ruthenium complexes are dramatioally different than the rhodium ones. They have been used to catalyze a variety of asymmetric hydrogenations, including the hydrogenation of enamides and alkyl and aryl-substituted acrylic acids. See Noyori, et al., Modern Synthetic Methods, 1989, 5, 115, incorporated herein by reference. Unlike the rhodium catalyzed reductions, ruthenium(II) carboxylate complexes possessing the BINAP ligand are efficient catalysts for the enantioselective hydrogenation of .alpha.,.beta.-unsaturated carboxylic acids. According to Ohta, et al, J. Org. Chem, 52, 3174 (1982), the carboxyl moiety of the substrate, and not other oxygen containing groups, is responsible for the stereoselective reaction. Asymmetric reductions of noncarboxyl-containing substrates by ruthenium complexes are inefficient.
The preparation of the BINAP-bearing ruthenium complexes, while not only sophisticated, is time consuming and expensive. Another disadvantage of using the BINAP ruthenium complexes for the asymmetric hydrogenation of olefins is the high solubility of the complexes in solvents used for hydrogenation. This characteristic increases the complexity of product purification and makes recycle of the expensive catalyst difficult if not impossible. Accordingly, it would be advantageous to be able to carry out these enantioselective transformations using an insoluble heterogeneous catalyst that could be easily separated for recycle (by filtration, for example) from the hydrogenation reaction mixtures, while at the same time providing a reaction product free of catalyst contamination. However, a generally recognized disadvantage of heterogeneous catalysis relative to homogeneous catalysis is a decreased selectivity. See, for example, H. Brunner in "Topics in Stereochemistry", Vol. 18, p. 134, Ed. by E. L. Eliel and S. H. Wilen, John Wiley & Sons, New York (1988).